Ferrous base article of manufacture



Jan. 1, 1929.

W. H. PHILLIPS FERROUS BASE ARTICLE OF MANUFACTURE;

Original Filed April 14, 1927 2 Sheets-Sheet WITNESS E5 Jan. 1, 192901,697,083

W. H. PHILLIPS FERROUS BASE ARTICLE OF MANUFACTURE Original Filed Ap gil14, 192'? n 2 Sheets-Sheet '2 h \D N 'C) n I\ E, 8 m U s \o g I gt: 2 ExY w s f m: F

wrmasszs I! mime F 2 2 9M. v 5' WM Patented Jan. 1, 1929.

WILLIAM H. PHILLIPS, OF SHAIJISR" TOWN-SHIP; ALLEGHENY" COUNT-Y,PEENNSYL- VANIA, ASSIGNOR TO MO PENNSYLVANIA, A CORP LYBDENUMCORPORATION ORATIO'NOF DELAWARE.

OF AMERICA, OF PITTSBURGH,

FERROUS BASE ARTICLE OF MANUFACTURE Continuation of application Serial NThis application is a continuation of:v my

pending application April 14, 1927.

In the accompanying (l SerialNo, 183 910, filed rawings, to whichreference will later be made,Fig. 1 is a photonncrograph of a toughnitrogen-ated case; Fig. 2 a photomicrograph of a brittle nitrogenatedcase; and Fig.

tain steel.

' The phenomena of surface nitrogenation whi ch of iron and steel, to

8 curves plotted from data obtained in the nitrogenatlon of a oer thisinvention pertains were extensivel investigated b *Fr as shown in hisarticle in is, 1271 1923),

found, among other thing according s, thatby subject Stahl und Eisen,

to which he ing plain carbon steel to ammonia-at an elevatedten'iperature there is formed on the surfaceot the steel a very hard andextremely brittle iron nitride layer of what he termed nitrides, Fe Nand Fe N.

composed largely the higher and lower iron He also found that in someinstances thicker andharder iron nitride layers are similarly formed onthe surface of low carbon alloy steels containing chromium, titanium,aluminum, manganese,

vanadium and silicon.

In research and commercial development work heretofore done on steel,the best results, practical standpoints, the use of relativelycontaining aluminum and ever, the nitrogenation of tended by seriousdisadvan spect to procedure and uit. As to procedure, it

the nitrogenation of from commercial and have been obtained by lowcarbon alloy steels chromium. Howthese steels is attages both with re.-

to the resulting prodrequires the exposure of an article formed ofaluminum-chromium alloy steel to an atmosphere containing nitrogen,derived from ie introduction of ammonia into a furnace containing thearti ole, at a temperature o f-about 540 C. for a period of fifty ormore hours to form on the steelacase about in thickness. As to theone-thirty-sccond of an inch product resulting from the nirtogenation ofaluminum-chromium steels, the layers y y have Satisfartory h termed ironnitride? ardness and thickness, but they are deficient in toughness andcohesiveness. These cedural, economic and ph disadvantages, proysical,incident to o. 183,910,,h1ed-Apri1' 14, 1927. This application filedJuly 13,

1928. S.eria1No. 292;f107I ofthe article, ivhichina'ddit'ion tobeing-hard are also tough, cohesive, and resist-ant to corrosion,andwhichartieles may be produced at less cost than prevailedprior to myinven illOll'. i

Previous disclosuresin this art appear to have drawn distinctionsbetween various types of ferrous materials adapted for'nitrogcnation; Myinvention is, on the contrary, applicable to alltypes of ferrousmaterial, whether iron or' steel, and 'whether cast, wrought or heattreatcd'in any manner used intheqproductionzofferrous articles- In thespecification and claims the term ferrous base is used for purposes ofdescription as comprehending all these materials in their variousconditions, g s

The invention ispredicated primarily upon my: discovery thatitsstated'objects may be attained by alloying molybdenum in substantialamount with the ferrous base used in the manufacture of articles whichare to be surface hardened by nitrogenation. processes. I havedis'coveredithat an, article formed of steel containing molybdenum as anessential alloyingv element may be provided with a hard, adherent, tongli cohesiveand corrosionness by subjecting it to nitrogenation atanelevated temperature for a period of time less thanhalf, and in some,cases less than a fifth offthat heretofore considered necessary fornitrogen atiou of Asjust used, and as further used herein and in theappended claims with referenceto nitrogenated cases the term, toughnessdefines that physical characteristic of the materi al otithe case. whichenables it to undergo nation whichfirmly adhere to the main bodyresistant layeror case of'satisfactory thick articles made of the steelsheretofore consideredlbest: suited forthe pur graphingthe specimen andthe shape of the;

plastictlow with little or no spalling in the immediate;vicinity of anindentation in the case formed by the Vickers"diamond indenter hardnesstester. Similarly, the term brittleness is used herein to definethatphysical characteristic of-the case which prevents it from undergoingplastic deformation and instead causes it to spall in the immediatevicin: ity of an indentation of the type'j'u'st referred to. Also, asapplied to the cases produced in accordance with the invention, the-termcohesiveness describes the ability of each difterential physical unit ofthe case material to cling to neighboring differential physical units. pI

These terms may be further understood and the benefits of the inventionshown in. part by refcrencc'to Figs. 1 and 2. Fig. l illustrates a toughcase formed by nitrogen'ation. of steel containing molybdenum inaccordance with the invention, but not containing chromium. 2illustrates a brittle nitrogenated case on a chromiunrsteel containingno molybdenum.

After nitrogenation, one surface of these samples was ground on a taperof one to twenty. The grinding was carried out slowly under a heavy airblast to retain the structure of the metal in an unchanged condition.Hardness measurements were then made on 'these. ground faces, beginningat the outer surface of the case, using a Vickers diamond indenterhardness tester.

The surface surrounding Fig. 2 was located about 0.00s inch belowtheouter riitrog-- enated surface. Itwill be observed that the material ofthe case was not capable of undergoing plastic flow as thus indented,but

instead spalled severely. The'outer fourthousandths of the case was allfound to be brittle, and the first tough material was found just belowthe region of Fig. 2. The hardness at this latter point, as measured bythe Vichers machine, was about 1000 oil.

The surface surrounding the in'dei shown in l, and characteristic of thetough cases produced by the invention, was located about 0.0005 .inchbelow the outer surface of the specimen as nitrogenated, and

the harcness was about 1000 Brinell, will be noted that the indentationis clear and sharp, and that the material of the case has beenplastically deformed without spalling.

This specimen was illuminated'in "the direction of the arrow 1. Thematerial in the region 2 shows plasticfiow. The'apparent lack of plasticflow in regions 3,4- and 5 is due to 'the'type of illumination usedin'photd material as piled up by plastic flow. Examination undertlemicroscope showed that" plastic tlow iad actually takenplace on alltne indentation; The curved,

on I

not 'craclrs, but are actually" nesagosa rial. This was proved byrotation of the specimen under the microscope upon which the linesdisappeared. I

The hardness tester referred to is pro terred because of its ability todistinguish between tough-and brittle nitrogenated cases. The Herbertpendulum hardness tester reveals no material ditt'erence in hardnessbetween these two specimens at the locations of Figs. 1 and 2. Thedifference in quality between these two cases, however is vital from thestandpoint of abrasive wear; and the differences shown bears out theproven principle that resistance to abrasive wear is best secured by acombination of hardness and toughness.

Isis my belief that the nitrogenation of ferrous materials heretoforenitrogenizedl have discovered that the addition of molybdenum to theferrous base so accelerates or activates the nitrogenation process as togive more satisfactory results than heretofore obtained, and in lesstime than was 'fOIlHEI'lY COHSlilGTBd necessary. My observation of thesenitrogenated articles leads me to believe that the beneficial action ofmolybdenumin the nitrogenation process is due to itsunique property ofreadily forming nitrides which dissociate readily according to areversible reaction, whereby, in the terrous base, molybdenum becomes aninternal transterrer of active nitrogen. lhus the molybdenum actsas anenergetic nitrogen collector, transferringnitrogen in a form in which itcan. be distributed at a rate and in a condition so. ii that it isreadily assimilable by the metal, perhaps as an easily diilusiblenitride, or as nitrogen in a particularly reactive or dififusible state;Molybdenum may,

and probably does, contribute to the hardness f in other ways, as byincreasing the hardness of the iron by forming additional solidsolutions which go hand in hand with increased hardness, I I

in addition to containing molybdenum as an essential alloying element,the ferrous base oifwhich the articles are formed may ducing hardness,strength or other desired characteristics in the product; and it may,

and usually does, contain the usual elements present as impurities andas incidental to crunn ercial processes or" manufacture of iron orsteel. in articles formed of steel, which embody the preferrediform ofthis invention,

the steel press contains aluminum or also contain other alloyingelements for pro 4 are normal.

reevgesa Molybdenum-from about 0.75 to about 1.00%. Siliconfrom about0.20 to about 0.25%. Phosphorus not over about 0.04%.

sulphur -not over about 0.04%. lhe manganese and silicon contents ofthis typical analysis arethose customarily found in steel as incidentaltoitS manufacture, and

the percentages of phosphorus and sulphur its a specific example of theinventioman article formedof steel containing:

lluminumuhl 2.2 arbon 0. 20% hlianganese in; M 0.55% lglolybdenum 0.89%"ilicon 0.18% Phosphor-11s-; below 0.044% 801 0110; M below 0.04%

carbon and 0.60 per cent of molybdenum after being subjected. tonitrogen at a tem perature of 540 C. for a period of twenty hours, wasfound to have a satisfactorily hardened surface layer. 7

The foregoing examples of nitrogenation according to the inventionillustrate the benefits to be obtained by the addition of molyb denum asan alloying element. ln order to obtain a case of the same depth withoutthe use of molybdenum, the steel would have to be subjected tonitrogenation for a much. longer period. My experiments indicate that asthe nitrogenation of any'steel by heating in. ammonia is prolonged, thesurface starts on transition from a tough toward a brittle condition.Thus, processes requiring long exposure are not only expensive, but tendto produce .an inferior product. I

As stated above, the alloy heretofore considered best for this purposeis a chromiumaluminum steel. In order to produce a sat isfactory caseupon chromhim-aluminum steels, they are, in practice, subj ected tonitrogenation for very long periods of time. As further illustrative ofthe advantage attendant upon the use of molybdenum, the followingexamples show the results obtained with sucha steel as compared withthose obtained with a steel of comparable analysis containingmolybdenum, but substantially no chromium: I lhe composition of thesteels was as follows, the amounts being given in percentages Steel 1nM0 or Mn s1 r s Steel 1%. is typical of that referred to as heretoforeconsidered the best for nitrogenation. Steel 'B is similar to steel A,but contains molybdenum instead of chromium.

Similar samples of each were nitrogenated under similar conditions ofammonia supply, and time, at 540 C. After nitrogenation, the a sampleswere cooled in the furnace and one surface of each sam le was ground toa. one to. twenty taper. a-rdness' measurements were made on thesegroundfaces, using a Vickers diamond indenter hardness. tester. Thedepth of the case was also measured with a taper gauge. a it was found.that after new. genation for 10 hours, the case formed on steel A wasabout 0.0055 inch thick, and its hardness expressed in Brinell number,was about 700 at'0.001 inch belowthe surface. Steel B, afternitrogenation for the same length of time developed a case about 0.01inch thick, the hardness of which 0.001 inch below the surface was about825. Samples of each of the above types of steel nitrogenatedv for.hours showed the following results.

lln still another test the nitrogenated case formed on a steelcontaining about 0.? 5 per cent of aluminum, and about 0.2; per cent of-molybdenum was tested for hardness in the manner described above. At0.001 inch below the surface the Brinell hardness was about 050, and at0.01 ineh'it was about 54.0. As compared with the cases produced whenlarger amounts of molybdenum are used, this, case was not quite as hardtoward the surface, but the transition from the case to the core wasmore gradual. Such a case is for many applications.

The examples just given clearly show one of the differences between theaction of molybdenum and that of chromium, in that steel B develops adeeper and harder case than steel A under the samenitrogenation.conditions. Another difference betweenthe action of thesetwo metals is shown in the type of case produced, as shown in F l and 2and described in connection therewith.

The examples cited show that nitrogenated cases, equal in hardness anddepth to the best previously obtainable, can be produced in materiallyless time. Likewise, the economy of time results in substantial savingin costs. The coatingsproduced when molybdenum is used are tougher andmore cohesive than those previously known in the art, and they adheresatisfactorily to the unnitrogenated core.

The invention is applicable to various forms of articles which require,or which it is desired to provide with, hard, tough, cohesive andcorrosion resistant surface layers. lVhatever the article may be, it isfirst formed, and before being subjected to the nitrogenation processmay be heat-treated to give its main body the desired physicalcharacteristics. The character of the case formed by the nitrogenationprocess may vary depending upon the heat treatment previously given thearticle. The article is then subjected to any suitable nitrogenationprocess, as for example by being heated in contact with any substancecapable of providing nitrogen in a form capable of causingnitrogenation.

of value In the examples cited, the articles were nitrogenated at orabout 540 C. The heating temperature may, however, vary according to thecomposition of the article, its shape, etc. I t has been stated by Frythat nitrogenation above about 580 C. causes the production of brittlecases, and that nitrogenation must be carried out below this temperaturein order to produce satisfactory nitrogenated cases. My experiments,however, have shownthat both the rate of ammonia dissociation and ofabsorption of nitrogen by the article may be at a maximum at highertemperatures, for example at or near 700 C. This is shown in Fig. 3 inwhich the curves represent data obtained in the nitrogenation of a steelhaving the following composition:

- Per cent. Aluminum l 2:50 Carbon 0.19 Manganese 0.64 Molybdenum 0.83-Phosphorus 0.010 Silicon 4 0.29 Sulfur 0.023

Curve 1 shows the effect of nitrogenation temperature upon the rate ofchange of the ammonia concentration in the efiluent gas from thefurnace. From 150 to 550 C. the change is gradual, from about 550 to 700C. the rate of dissociation increases rapidly, and above 700 C. itdecreases quickly. Curve 2 represents the effect of temperature upon thegain in weight, plotted as milligrams per square centimeter of exposedsurface. This curve follows the same general outline as curve 1,reaching a, peak at 700 C.

The curves of Fig. 3 show that with this steel, which falls within thetypical analysis referred to hercinabove, the rate of ammoniadissociation and of nitrogen absorption are at amaximum at. or about 700C. The ferrous base articles-containing molybdenum according to theinvention may be treated at temperatures above 580 C. to produce hard,tough, cohesive, and adherent cases, contrary to the prevailing belief.In many instances it may be desirable to operate at temperatures nearthe peak of the curve typical of the alloy being treated, because of theincreased rate of nitrogenation, and the consequent economies.

Nitrogenated cases are also resistant to corrosion, offering protectionagainst the rusting action of the atmosphere. Ordinary carbon steelappears to become sufficiently corrosion-resistant for most purposesafter nitrogenation, but the case is very brittle and spalls or chipsoff easily. I have found that the tough and coherent nitrogenated caseproduced on the ferrous base articles according to the invention issatisfactorily resistant to a variety of types of corrosion.

For corrosion resistance, hardness is not usually of prime. importance,and I have found that cases possessing satisfactory corrosion resistanceand good physical properties may be obtained by nitrogenation atelevated temperatures where the nitrogen absorption is at a maximum,such as at 700 C. Thus nitrogenation for short periods, three hours forexample, at 700 C. will give suflicient nitrogen absorption to provide acase which is satisfactorily corrosion-resistant for a wide variety'ofapplications. This decreased time of treatment for such purposes is. ofcourse of great commercial importance.

The mechanism of hardening by nitrogenation is obscure, but it isprobable that the nitrogen acts by forming a hard nitride or nitrides,and that this or these, being hard, confer that property to the case.The

nitrogen hardening may also be due to diffusion of such nitrides to formsolid solutions, which are generally of increased hardness, as is wellknown; and also nitrogen itself may diffuse and form solid solutions.Any, or all, of these phenomena may occur, and others may contributewholly or in part.

Although molybdenum, the use of which characterizes this invention, mayincrease the hardness by forming solid solutions, its

chief functionis to accelerate nitrogen fixation and transference 1na'manner peculiar to this metalitself'. My observations make it seemprobable that the mcehanism of hardening is about as follows. Iron, whenin a massive state, combines with or dissolves nitrogen slowly, so thatthe hardening process is slow. at best, and as previously mentioned, thecases formed'on iron or low carbon steel are very brittle. While thephysical characteristics of nitrogenated cases are claimed to beimproved somewhat by. the

- use of chromiumwhich characterized the alloys previously consideredbest suited for hardening action.

this purpose, yet even when chromium is used the nltrogenatlon periodhas been long,

because the chromium nitrogenation prod-.

ucts form slowly and are relatively stable. The stability of, suchproducts materially delays the progress of .nitrogenation, because theydo not readily give up nitrogen to the metal, or otherwise assist inhastening the Molybdenum, however, take up nitrogen in the formliberated in the dissociation of ammonia, and furthermore,

' the nitrogen thus taken up is apparently readily liberated in a formin which it diffuses and passing progressively inward, the molyb-.

nitrogen-and liberates it, and" I may,

most readily through the solid metallic matI'lXk'. Some of the nitrogenthustaken up is 7 set free, and diffuses inwardly into the surroundingmetal to form solid solutions and nitrides, It appears also that whenthe concentration analogous in character and hardening action,

has become suflicient, a compound to iron carbideis deposited. Thiscompound cementite, because of its appearance under the microscope, Atthe same time the molybdenum'below'the surface. has been receivingnitrogen by this diffusion from that nearer the surface, and it in turnliberates nitrogen for rediffusion. Thus beginningatthe surface,

'd'enum takes u by virtue of t much more rapi where the sluggish directdiffusion of nitrogen itself was probably the chief or only factor; Inaddition the faster or more ready is action the nitrogenation isdiffusion of the nitrogen or nitrides'caused. by molybdenum, producessuperior results y appears to readily for lack of a better term becalled mtride d than was heretofore known .rosion. vIIItQStImOIIywhereof, IY- sign my name-ff because more uniformdistribution is secured.

From the foregoing it sufliciently appears that according to thisinvention there are provided at reduced cost of manufacture ferrous basearticles which have improved surfacelayers produced by nitrogenation. Ashas been explained, these advantages are attained by the addition ofmolybdenum to iron combined with carbon, and particularly by theaddition of molybdenum to steel which preferably, also containsaluminum.

' No claim is made herein for the method of forming a nitrogenated caseon-the surface of a ferrous basealloy, the method herein disclosed beingclaimed in a divisional application.

According to the provisions of the patent statutes, Ihave explained theprinciple of my invention, and have specifically described What I nowconsider to represent its best embodiments and the manner in which itsadvantages may be attained. However, I desire to have it understoodthat, within the scope of the appended claims, the invention may bepracticed otherwise than as specificallydescribed. Furthermore, I desireto have. it imderstood that the foregoing explanations of about 4 percent of molybdenum as a characterizing'element, and having a hard,tough,

coherent, and adherent nitrogenated surface. 1 3; An article ofmanufacture composed of a steel containing from a substantial amount upto about 4.0 per cent of'molybdenum as a characterizing element,'andabout 0.1,to 4 per cent of aluminum, and having'ahard,.

tough, coherent and adherent nitrogenated is resistantto atmosphericcor- I surface which wninmnni rnninirs

